Hey. I love to duel as a SP, so I basically spend most of my wow time dueling. There are only two classes that I can't beat, warriors and hunters.
Dueling mages is quite easy. I am full pvp (normal blue weapon) and I win most of the time versus mages with pvp weapons. You didn't specify if they were frost or fire. My tactic is to get three orbs before the duel, I know it's unfair but since I have no pvp weapons, it evens it out.
You got to have the opener. If he uses invisibility, you should use halo, followed by a fade (phantasm) so he won't silence/poly you.
I put SW:P and start dispelling, as soon as I dispel his ice barrier I cast devouring plague + VT. From then on, you should always use spectral guise to survive the deep freeze and use the rest of your cooldowns in a good timing. =)
This is how I play, it might not be the best way... but it works for me. =)